1. Technical Field
The present invention generally relates to communications systems including remotely located communications terminals and, more particularly, to a method and apparatus for defeating the use of illegal communications terminals in the communications systems.
2. Description of the Related Art
In order to enhance revenues, suppliers of video programming (primarily cable TV systems) offer special programming for which they charge fees. In order for such systems to be profitable, the video signal must be modified so that a standard television receiver will not produce a viewable picture--otherwise no one would pay the extra charge. This is known as video scrambling. The cable system operator then supplies an extra piece of equipment to those who have paid the extra fee. Among other functions, this equipment, known as a set-top terminal (STT), reverses the scrambling process. The "descrambling" results in a viewable picture.
A composite video signal can be viewed as consisting of two types of information, the actual video information and synchronization information ("sync"). The sync can be considered to be further divided into horizontal and vertical sync. The purpose of the sync is to serve as a timing reference so that the TV or other display device can reconstruct the desired image in a viewable form. One horizontal line of normal NTSC video is shown in FIG. 1. It can be seen that the line is characterized by a horizontal sync pulse 101, represented by the sync tip pulse or the most negative state of the video signal. This pulse normally lasts 4.7 microseconds and is repeated 15,734 times each second in the NTSC television system used in the United States and many other countries. Following the sync pulse, the signal voltage returns to the blanking level which is used as a reference value. By common convention, the amplitude of the blanking level is considered to be 0 IRE, a unit of measure adopted by the Institute of Radio Engineers (now the IEEE). The sync tip is at a level of -40 IRE, while peak white is at a level of +100 IRE. This maximum normal excursion of the TV signal of 140 IRE is conventionally equated to 1 volt peak-to-peak, though other voltage levels are sometimes employed internal to a particular piece of equipment. Following the return to a blanking level after a sync tip portion, and after a delay known as the breezeway 102, the color burst 103 occurs. The color burst is composed of eight cycles (nominally) of the color subcarrier, 3.58 MHZ in NTSC transmission. The amplitude, and more importantly, the phase of the color burst are essential to proper recovery of the color information, as is well understood by those in the art. After a delay following the color burst, the active video interval, indicated generally as 107, begins. The time from the end of the sync pulse to the beginning of active video is called the back porch 106. The end of the active video defines the front porch 104, shown twice to emphasize that the signal repeats. The entire interval from the beginning of the front porch to the end of the back porch is collectively known as the horizontal blanking interval (HBI) 105.
The active video interval 107 actually consists of various voltages representing the brightness (luminance) of the image, plus a color subcarrier (not shown) which carries color saturation ("purity") information as amplitude modulation and color value ("tint") represented by its phase with respect to burst 103. This pattern of sync and active video is repeated for 252.5 lines (including vertical blanking). This number of lines constitutes one "field" and is followed by an interleaved second field. The two fields together make up a "frame", or one complete picture. Typically, the frame rate is 30/second, i.e., frames are produced 30 times each second.
A normal television or other video device recovers sync in the following manner. The most negative portion of the waveform is clamped to a known level, and a threshold is established a known distance above the clamp level. The distance above the clamp level is chosen so that video is well above the threshold, while the sync tip is well below the threshold. This is illustrated in FIG. 2.
The most common methods of video scrambling rely on changing the sync levels in the video waveform so that the sync detector circuit in a standard television receiver (or set-top terminal) will be unable to find the sync, thus resulting in an unviewable picture. The video signal has extra information "hidden" in it which allows an authorized STT to determine what must be done to restore the sync to the appropriate levels. This extra information is usually provided as AM modulation on the sound subcarrier or during the vertical blanking interval (VBI) of the composite video signal. The extra information is normally encrypted in some fashion in order to make it more difficult for people to build illegal descramblers (pirate boxes). Examples of systems which rely on changing sync levels for scrambling are shown in U.S. Pat. No. 5,319,709 to Raiser et al. (sync suppression scrambling) and U.S. Pat. No. 4,924,498 to Farmer et al. (video scrambling employing split sync pulses).
Various attempts have been made to develop descramblers for descrambling scrambled video transmissions without payment to the cable operator providing the video transmissions. Recently, a new type of illegal descrambler has been developed which detects the edges of sync signals embedded in scrambled video signals, rather than the levels of these embedded sync signals. Using these detected sync edges, the illegal descrambler generates its own version of a sync signal, which it then reinserts into the video, thereby permitting the scrambled video to be unscrambled. Current scrambling methods which rely on changes in sync levels to prevent unauthorized viewing of scrambled video are ineffective against these pirate boxes which are sensitive to the edges of sync in the video waveform.